Recently, photo-alignment has been successfully introduced in large scale production of liquid crystal displays (LCD) and anisotropic optical films for various applications, such as 3D-converter films, also known as film patterned retarders, for passive 3D television and monitors. In each of above applications, thin photo-alignment layers are employed to align liquid crystals. In case the photo-alignment layer is used inside a liquid crystal panel to align the switchable liquid crystals, the alignment property of the photo-alignment layer has to be maintained over the lifetime of the display since the liquid crystal material has to be realigned each time it has been switched due to interaction with an applied electrical field. In case of anisotropic optical films the liquid crystals are cross-linked after they have been aligned by the photo-alignment layer. Specific embodiments can, for example, be found in U.S. Pat. No. 6,717,644.
Compared to conventional alignment of liquid crystals by brushed surfaces, the photo-alignment technique has many advantages, such as high reproducibility, alignment patterning and suitability for roll to roll manufacturing. In addition, photo-alignment can be applied on curved surfaces, such as lenses, since the light which generates the alignment in the photo-alignment layers can follow the surface modulation, which is not the case for most of the alternative alignment methods. In the state of the art photo-alignment technique thin layers of photo-alignment materials are applied to a substrate, such as a glass plate or a plastic foil. Since the alignment information is transferred by the surface of the alignment layers, its thickness is less important and device manufacturers choose low thickness to reduce material costs. A typical thickness of photo-alignment layers in the state of the art is around 100 nm or less. This is in particular the case for application as alignment layers in LCDs, where thicker layers have the disadvantage that they lead to an increase of the effective threshold voltage for switching the LCD.
As long as the substrates are flat or slightly curved there are different standard coating techniques which can be used to homogeneously apply the photo-alignment layer. However, if a photo-alignment layer has to be applied to a substrate comprising smaller structures, for example microstructures, such as micro lenses or micro-prisms, or structures exhibiting abrupt changes of the shape, such as rectangular structures, coating of a thin homogeneous layer is more complex and depending on the specific application, may even be impossible.
A further drawback of the state of the art photo-alignment technique is that the substrates which are used as a support for materials which need to be aligned, such as liquid crystals, have first to be coated with a thin photo-alignment layer, which increases costs and manufacturing time and reduces the production yield.